Tube rolling plant

ABSTRACT

The present invention teaches rolling a seamless tube, typically with a large diameter. The system comprises a furnace for heating billets produced by continuous casting. The system comprises a piercing mill for longitudinally piercing the heated billets to obtain a pierced blank. The system comprises an expanding-elongating mill for expanding the pierced blank diameter and for elongating the pierced blank to obtain a semi-finished tube. The system comprises a continuous main rolling mill with adjustable rolls for mandrel-rolling a semi-finished tube. The system comprises a fixed-roll extracting-reducing mill positioned downstream of the main rolling mill. The extracting-reducing mill extracts the semi-finished tube from the mandrel, reducing the diameter to a predetermined value close to that desired for the finished tube. The system comprises an adjustable-roll sizing mill to adjust the radial position of the rolls and define the diameter of the outgoing tube.

CLAIM OF PRIORITY

This application is a U.S. Continuation Application of InternationalApplication No. PCT/IB2010/050017, filed Jan. 5, 2010, which is herebyincorporated by reference in its entirety.

FIELD

The present invention relates to a system and method for production ofseamless tubes, in particular for the production of large-diameterseamless tubes. The expression “large diameter” is understood here andbelow as meaning diameters of between 457.2 mm and 711.2 mm (i.e.between 18″ and 28″).

BACKGROUND

The production of small-thickness large-diameter tubes is performed atpresent by means of deformation of metal sheets, thereby obtaininglongitudinally welded tubes. This tube production technology, althoughwidely used, is not without drawbacks. Firstly, tubes with only arelatively small wall thickness may be obtained. Second, the metalsheets from which the tubes are obtained may have a maximum thickness ofthe order of 30 to 35 mm. A further disadvantage of welded tubescompared to seamless tubes is that the former have a smaller mechanicalstrength and corrosion resistance, in particular along the weld.

As an alternative to welded tubes, it is also possible to produceseamless tubes. A type of rolling mill called “pilger mill” is used in aknown manner for the production of large-diameter seamless tubes. Thisrolling mill uses grooved rolls with a variable groove depth along theircircumference. The

central section of the roll is therefore cam-shaped, i.e. not circular.Processing of the tube in this rolling mill requires continuousdisplacement of the blank backwards and forwards along the rolling axis.

Although being at present the only machines used for the industrialproduction of large-diameter seamless tubes, pilger mills have a numberof drawbacks.

Firstly, it is a fairly slow machine. For example, the typicalproduction output of such a machine is about 12 to 15 tubes per hour,compared to the 60 or more tubes produced by normal continuous rollingmills.

Moreover, the pierced blank used in the pilger mill cannot be obtainedfrom an ordinary continuous casting billet. In fact, because of itsspecific characteristics, a pilger milling results in considerableelongation of the pierced blank. This elongation of the pierced blankmust necessarily be compensated by a substantial reduction in thediameter. In view of these technological constraints, the production oflarge-diameter tubes also requires large diameters of the piercedstarting blanks, which consequently cannot be obtained from ordinarycontinuous casting billets. In fact, the maximum diameter of standardbillets nowadays is no more than 500 to 550 mm and is thereforeinsufficient. Larger-diameter billets could be obtained from continuouscasting plants designed with specific dimensions. The quantity oflarge-diameter billets normally required by the market does not justify,however, the huge investment needed for the construction of such aplant.

Therefore, the pierced blanks used for rolling in a pilger mill musthave diameters of up to about 950 mm and consequently must be obtainedfrom ingots which have sufficiently large diameters. The person skilledin the art knows that an ingot, for technological and production-relatedreasons, costs up to 30% more than a billet. Moreover, the quality of aningot is inferior to that of a continuous casting billet. In fact, aningot does not have very uniform characteristics and the wasteassociated with this production method, namely the sprue or riser,significantly penalizes the manufacturing costs.

The waste associated with the tail end of the tube rolled in a pilgermill is also considerable. This rolling process in fact produces atypical “bell”, i.e. an end part of the tube which cannot be rolled andwhich must inevitably be cut off and discarded. Considering, therefore,the starting material and the type of process, the pilger mill rollingmethod has overall a relatively low output.

A major problem associated with the use of the pilger mill is moreoverthe poor quality of the finished tube. The type of working processdescribed above and the geometrical form of the incoming pierced blankare such that the walls of the finished tube are somewhat irregular.This characteristic of the tubes obtained by means of a pilger millconventionally has not been regarded as a problem. Nowadays, however,with the much higher quality standards which can be achieved withcontinuous rolling mills, this characteristic is increasingly beingregarded as a defect, in particular in view of the high cost of theproduct.

In the past another technology for the production of large-diameterseamless tubes has also been used. This technology is based on a machineknown as an “expander”. An expander basically allows deformation of atubular blank so as to obtain a finished tube with a larger diameter,smaller wall thickness and length substantially the same as that of thetubular blank. The percentage increase in diameter, or expansion,typically obtained with an expander may be reckoned as having a value ofup to 60%. The maximum expansion which can be obtained by the expander,however, depends on the wall thicknesses of the incoming tubular blank.

Typically, during processing with an expander, the metric weight of theincoming blank and of the outgoing product remains substantiallyunvaried. For this reason, in order to obtain large outgoingthicknesses, it would be necessary to start with incoming thicknesses solarge that they would be difficult to achieve in practice. Moreover,even if it were possible to achieve these thicknesses for the blankentering the expander, the typical helical scoring present on the insideof the outgoing blank would be very marked and therefore unacceptable.

In an expander, in fact, the rod which supports the plug inside the tubeoperates under compression. It is known that this stressed conditionplaces a limit on the maximum load, this limit being fairly low in orderto prevent the rod being affected by buckling resulting from thecompressive stress and to ensure correct set-up of the machine andprecise control of the process. For this reason, the large wallthicknesses, responsible for high compressive loads on the plug, requirelow percentage expansion values.

Moreover, high expansion of the diameter with large wall thicknessesresults in increasing unevenness inside the tube leaving the expander.This unevenness, in the form of helical scoring, can only be eliminatedwith difficulty by the subsequent machining operations.

This technology has not been very successful on account of theconsiderable number of drawbacks associated with it. First of all, theproduction of tubes was performed using tubular blanks which were also,in practice, finished tubes. In view of the typical expansion ratios ofthe expanders, in order to obtain a finished tube with a diameter of28″, it was necessary to use initially a blank with a diameter of 18″.

At the time when expanders were widely used, the 18″-diameter tubes wereobtained by means of the already mentioned pilger mill sinceretained-mandrel rolling mills were not yet available for suchdiameters. Obviously the poor wall quality of the starting tubesdirectly affected the quality of the finished tubes. The expanderprocessing step certainly could not improve the quality and, on thecontrary, also introduced further defects. This was one of the reasonswhy this technology was in fact abandoned in favor of higher-capacitypilger mills able to produce directly in a single pass tubes with thedesired diameter and of comparable quality.

A further disadvantage of the technology associated with an expanderconsisted in the fact that the tubular blanks had to be heated in aspecial furnace before processing. This heating stage always proved tobe somewhat critical. The temperature of the tubes, in fact, had to beincreased from the room temperature typical of warehouses to the 1200 to1250° C. required for working. This heating operation, therefore,increased considerably the amount of time and the costs involved.

In particular, in order to achieve a temperature which was as uniform aspossible on the tube and sufficiently high to allow optimum workingthereof, the heating stage had to be prolonged, in particular in thecase of large-thickness blanks. The longer the heating stage, thegreater the production of oxides occurring inside the tube. These oxidesthen had to be removed in order to improve the workability of the tube,reduce the internal defects and ensure a minimum quality of the finishedproduct. Removal of the oxides is still a fairly complex operation andinvolves the use of a saline solution. It is therefore a criticaloperation in particular from the point of view of environmental safety.

The problems mentioned above in connection with the production ofstandard steel tubes are exacerbated even more during the production oftubes made of high-alloy steels, for example steels with a chromiumcontent of 10% or more. The mechanical characteristics typical of thesesteels result in a reduced deformability of the material and, therefore,as regards the expander, increase the compressive stresses acting on theplug during operations involving a high degree of expansion. Moreover,high-alloy steel tubes are commonly required by the market inmedium-to-large wall thicknesses, thereby further increasing the workingdifficulties associated with the expander.

The production of large-diameter seamless tubes could also be performedby means of a continuous rolling mill of the type commonly used formedium-diameter tubes. In this type of machine, the tube is rolled bypassing it through a series of rolling stands (or stations) eachcomprising two or more rolls, usually three rolls. The rolling standsare normally five or more in number and the position of the rolls isadjustable in the radial direction. This type of working operationrequires a mandrel arranged inside the tube so as to be able to opposethe radial thrust exerted by the rolls during rolling. In order to exertthis opposing action, the mandrel must be extremely rigid in the radialdirection. Moreover, in order to ensure a high quality finish on theinner surface of the tube, the mandrel must have an outer surface whichis as smooth as possible. Because of this requirement, it would beextremely difficult to manufacture mandrels consisting of several partsjoined together. The joining zone is in fact necessarily characterizedby an irregular surface. Moreover, this zone would be too delicate towithstand adequately the radial rolling pressure.

It is known, in this sector, to use a retained mandrel: the mandrel isaxially constrained and is retained so as to advance at a controlledspeed. This solution has a major drawback. The single section of themandrel, although being braked, is advanced axially along the rollingmill. The single section of the mandrel is thus engaged in succession,under maximum deformation conditions, within all the rolling stations.Inside the rolling stations, the mandrel is subject to high thermal andmechanical stresses due to the deformation energy and the frictionproduced by the sliding contact of the tube material. The passagethrough more than one rolling station therefore causes a significantincrease in the mandrel temperature, thereby resulting in the need toprovide several mandrels which are identical to each other such thateach one of them may be suitably cooled at the end of rolling and thenlubricated for the next rolling cycle.

In addition to this, it must be considered that the individual mandrelmust be made of a particularly high-quality material in order towithstand the stresses typically arising during rolling. Obviously theoutlay required for a mandrel depends on its dimensions. The typicallengths of retained mandrels are in fact such that the manufacture of anentire set of large-diameter (i.e. more than 20″) mandrels required forconventional continuous rolling is disadvantageous from the point ofview of costs.

The object of the present invention is therefore to overcome at leastpartly the drawbacks mentioned above with reference to the prior art.

In particular, an aim of the present invention is to provide a systemand a method for the production of large-diameter seamless tubes.

Moreover, an aim of the present invention is to provide a system and amethod for the production of tubes with a wide range of wall thicknesses(from small to large).

Furthermore, an aim of the present invention is to provide a system anda method for the production of tubes made of different types of steel,including both carbon steel and high-alloy steel.

Furthermore, an aim of the present invention is to provide a system anda method by means of which it is possible to obtain finished tubes ofsuperior quality compared to those currently available on the market.

Finally, an aim of the present invention is to provide a system and amethod which are able to also produce medium-diameter seamless tubes,i.e. those with a diameter of between 339.7 mm and 508 mm (13.⅜″ to20″).

Some of the above mentioned objectives and aims are achieved by a systemas claimed in claim 1 and by a method according to claim 11.

The characteristic features and further advantages of the invention willemerge from the description, provided herein below, by way of a numberof embodiment, provided as non-limiting examples, with reference to theaccompanying drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram representing a system according to theprior art;

FIG. 2 shows block diagram representing a system according to thepresent invention;

FIG. 3 schematically shows the detail of an expander-elongator used inthe system according to the present invention;

FIG. 4 schematically shows the continuous main rolling mill used in thesystem according to the present invention;

FIG. 5 schematically shows, in the form of a thickness/diameter diagram,the various types of tubes which can be produced according to thepresent invention.

DETAILED DESCRIPTION

The seamless tube rolling system according to the present inventioncomprises one or more of the following components:

(1) a furnace for heating billets produced by means of continuouscasting;

(2) a piercing mill for piercing longitudinally the billets so as toobtain a pierced blank;

(3) an expanding-elongating mill for expanding the diameter of thepierced blank and for elongating the pierced blank so as to obtain asemi-finished tube;

(4) a continuous main rolling mill of the type comprising stands withtwo or more rolls, in which the radial position of the rolls isadjustable, for performing rolling of a tube on a retained mandrel;

(5) a fixed-roll extracting-reducing mill positioned downstream of themain rolling mill and in series therewith, the extracting-reducing millbeing designed to extract the tube from the mandrel and to define apredetermined value for the diameter of the tube;

(6) a sizing mill for defining the diameter of the finished tube, thesizing mill being of the type in which the radial position of the rollsis adjustable; and

(7) a cooling bed.

Moreover, the system according to the invention comprises a by-pass linefor feeding, where possible, the pierced blank leaving the piercing milldirectly to the main rolling mill, thereby avoiding theexpanding-elongating mill.

The components of the system, according to a number of embodimentsthereof, are described below. This description is, in places, somewhatbrief since some of the components of the system are known per se to theperson skilled in the art, even though arranged and used in differentways.

The billet furnace is a furnace conventionally used in the sector andwell known to the person skilled in the art.

The billet piercing mill (or piercer) may be a standard conical-rollpiercer comprising two inclined-axis rolls which act on the outersurface of the billet and a plug which is inserted in the middle of thebillet along the hole.

From a conceptual point of view, the expanding-elongating mill (orexpander-elongator) is a machine quite similar to the piercer. For thisreason, in accordance with certain embodiments of the invention, thepiercer and the expander-elongator may be the same machine preset in twodifferent configurations, as described in greater detail below.

The attached FIG. 3 shows a diagram of an inclined-axis conical-rollmill in the configuration where it is designed to perform the functionof an expanding-elongating mill. The mill, denoted in its entirety by10, comprises a pair of rolls 12 of variable conicity rotating aboutrespective axes. The rotation axes of the rolls 12 are inclined relativeto each other. The expander 10 also comprises an ogive-shaped plug 14connected to a rod 16. The rod 16 may be arranged as shown in theattached FIG. 3 so that it is subject to compression during rolling.Alternatively, the rod 16 may advantageously be arranged on the oppositeside of the ogive-shaped plug 14 so as to be subject to a pulling force.

The pierced blank 20 is rotated about its axis and pushed against theogive-shaped plug 14 in the direction of the arrow f shown in FIG. 3. Ascan be seen from the diagram in FIG. 3, the combined configuration ofthe rolls 12 and the ogive-shaped plug 14 define a travel path alongwhich the material of the pierced blank must flow. The travel movementalong this path causes the desired deformation consisting of anexpansion-elongation.

In particular, in accordance with certain embodiments, the profiles ofthe rolls 12 and the ogive-shaped plug 14 are defined so that a part ofthe travel path causes expansion of the diameter and elongation of thetube and the remainder of the travel path instead results in the desiredexpansion of the tube diameter. Obviously, a reduction in the thicknessof the tube wall is also obtained along the entire travel path.

For example, according to one embodiment, the profiles of the rolls 12and the ogive-shaped plug 14 are defined so that the first approximatelytwo thirds of the travel path cause simultaneously reduction in the wallthickness, expansion of the diameter and elongation of the tube. Theremaining approximately one third of the travel path instead causesreduction in the wall thickness and the remaining desired expansion ofthe tube diameter.

According to one embodiment, the expanding-elongating mill causes anexpansion of the diameter equivalent to about 35% and a tube elongationby a factor of about 1.7.

In accordance with certain embodiments, the rolling mill shown in FIG. 3is designed to be rapidly reconfigured so as to perform alternately thefunction of a piercing mill and the function of an expanding-elongatingmill. In particular, the transition from one configuration to the othermay be achieved by means of a different orientation of the axes of therolls 12 and by means of a different form of the ogive-shaped plug 14.

In this case, the pierced blank leaving the machine configured as apiercing mill is processed again by the same machine reconfigured to actas an expanding-elongating mill. Only after the second pass, thesemi-finished tube is fed to the main rolling mill.

The use of such a machine which can be reconfigured, althoughcomplicated and per se somewhat costly, may in any case be advantageouscompared to the use of two different machines of the conventional type.

The main rolling mill, which is of the type with mill-stands having twoor more adjustable rolls and a retained mandrel, may be for example ofthe type described in international patent application PCT/EP99/01402filed in the name of Demag Italimpianti S.p.A. and published undernumber WO 99/47284. Preferably, the main rolling mill according to theinvention comprises stands with three rolls.

According to one embodiment of the invention, the main rolling millcomprises four rolling stands arranged in succession. This solutionconstitutes a particularly convenient adaptation of conventional rollingmills comprising two or more adjustable rolls. These rolling mills infact usually comprise five or more rolling stands arranged insuccession.

The feedback controls as to the position of the rolls in the mainrolling mill, based on the tube thickness, and in the sizing mill, basedon the tube diameter and temperature, may advantageously be of the typedescribed in patent application IT MI2009A001085 filed by the sameapplicant on 19 Jun. 2009.

In accordance with certain embodiments of the system according to theinvention, the main rolling mill is characterized in that it uses a slowmandrel. In the present description, the term “slow mandrel” isunderstood as meaning a mandrel which is retained so that none of itssections is subject to the action of two successive rolling stations.More particularly, with reference also to the attached FIG. 4, thefollowing equation is applicable:

V _(m) <d/T ₁

where V_(m) is the speed of the mandrel 32; d is the minimum interaxialdistance between two successive rolling stands 34; and T_(l) is therolling time. Also applicable is the equation:

T _(l) =L _(t) /V _(t)

where L_(t) is the length of the tube 20 and V_(t) is the axial speed ofthe tube 20 along the rolling mill 30.

From the above it can be understood that the mandrel 32, required foroperation of the main rolling mill 30 used in the system according tothe invention, may be relatively short. The minimum length required willin fact be equal to the overall interaxial distance D (i.e. the distancebetween the first and last rolling station) increased by thedisplacement S_(m) which the mandrel 32 performs during the rollingtime: S_(m)=V_(m)T_(l). The above equations also give the followingvalue: S_(m)<d.

Considering the embodiment of the main rolling mill 30 according to theinvention schematically shown in FIG. 4, the overall interaxial distanceD is fairly short because the rolling mill comprises a small number ofrolling stands 34, in the specific case only four stands. Moreover theextremely low speed of the mandrel V_(m) also allows a smalldisplacement S_(m) of the mandrel 32. Considering the average valuestypically assumed by the variables indicated above, the minimum lengthof the mandrel 32, equivalent to D+S_(m), will be between about 5 and 6metres. This length is such that the mandrel 32 may be manufactured at arelatively low cost, despite the large cross-sections required for theproduction of tubes with a diameter of up to 28 inches.

Moreover, since each individual section of the mandrel is subject to theaction of only one rolling stand, the overall amount of heating of themandrel during the process is limited. Due to this, it is possible tomanufacture the mandrel using materials which are less expensive thanthose used for conventional faster mandrels, without any negativeconsequences.

Moreover, as can be noted from the attached FIG. 4, the three interaxialdistances separating the four rolling stands 34 are not all the same.The first interaxial distance d, which separates the first stand fromthe second stand, and the third interaxial distance d, which separatesthe third stand from the fourth stand, are substantially the same.However, the second interaxial distance, which separates the secondstand from the third stand, is greater than the other two distances. Amini support stand 36 for the mandrel 32 is in fact positioned betweenthe second rolling stand and third rolling stand since the mandrel wouldotherwise be extended cantilever-like along the rolling mill 30.

It is assumed, as in FIG. 4, that the second interaxial distance isgreater by a distance j than the other two distances; each of thesections of the mandrel 32, during the entire rolling process, travelsat the most along a section having a length S_(m)<d. In connection withthe second interaxial distance, it is therefore possible to identify asection of the mandrel 32 with a length at least equal to j which doesnot undergo any rolling either by the second stand or by the thirdstand. This section of length j is therefore available for performing ajoint 33 between two sections 32′ and 32″ of the mandrel 32. Withreference to the example considered above, the two sections 32′ and 32″of the mandrel 32 would each have a length of between about 2.5 and 3metres. With these lengths, it is possible to simplify drastically themanufacturing and management of the mandrel 32, even in the case of thelarge diameters considered here (greater than 24 inches).

Moreover, using a built-up type mandrel, it is possible if necessary toreplace only the worn portion. In contrast, when using conventionalnon-built-up mandrels, the entire mandrel must be replaced even if it issubject to only localised wear. This possibility offered by the built-upmandrel reduces significantly the operating costs of the rolling mill.

The solution employed here consisting of a slow built-up type mandrel,together with the overall smaller dimensions described above, thus makeit possible to provide a main rolling mill for large-diameter tubeswhich is extremely competitive on the market.

The fixed-roll extracting-reducing mill has the function of extractingthe semi-finished tube from the mandrel and of reducing the diameter ofthe semi-finished tube to a predetermined value which is close to thedesired value of the finished tube.

According to one embodiment of the system, the extracting-reducing millmay be replaced by a combination of machines which together are designedto perform a similar function. For example, the extracting-reducing millmay be replaced by the combination consisting of an extracting mill,specifically intended to extract the tube from the mandrel, and areducing mill, designed to define a predetermined diameter of thesemi-finished tube.

In accordance with certain embodiments of the invention, the system alsocomprises, downstream of the extracting-reducing mill, means formeasuring the wall thickness of the semi-finished tube. In theseembodiments, the main rolling mill is able to adjust the radial positionof the rolls depending on the measurement of the wall thickness of thetube leaving the extracting-reducing mill.

In accordance with certain embodiments of the invention, the sizing millcomprises means for measuring the temperature of the incoming tube andmeans for measuring the diameter of the outgoing finished tube. In theseembodiments, the sizing mill is able to adjust the radial position ofthe rolls depending on the measurement of the temperature of theincoming tube and the measurement of the diameter of the outgoingfinished tube.

The invention also relates to a method for performing the rolling ofseamless tubes. The method according to the invention comprises thefollowing steps:

(1) heating a billet produced by means of continuous casting;

(2) longitudinally piercing the heated billet so as to obtain a piercedblank;

(3) expanding and elongating the pierced blank so as to increase itsdiameter and length and reduce its thickness;

(4) rolling the semi-finished tube in a main rolling mill so as toobtain a tube, the main rolling mill being of the continuousretained-mandrel type comprising stands with two or more adjustablerolls;

(5) extracting the tube from the mandrel;

(6) defining a predetermined value for the diameter of the finished tubein a sizing mill of the type comprising adjustable rolls; and

(7) cooling the finished tube.

In accordance with other embodiments, the method also comprises thesteps of:

measuring the wall thickness of the tube after extraction from themandrel; and

adjusting the radial position of the rolls of the main rolling milldepending on the measurement of the wall thickness of the tube.

In accordance with other embodiments, the method also comprises thesteps of:

measuring the temperature of the tube entering the sizing mill;

measuring the diameter of the tube leaving the sizing mill; and

adjusting the radial position of the rolls of the sizing mill dependingon the measurement of the temperature of the incoming tube and themeasurement of the diameter of the outgoing tube.

In accordance with certain modes for implementing the method, the stepof longitudinally piercing the billet is performed by means of a machinewhich can be reconfigured. According to these modes of implementation,the method also comprises, following the step of longitudinally piercingthe billet to obtain a pierced blank, the step of reconfiguring themachine so that it is adapted for expanding and elongating the piercedblank so as to increase its diameter and length, while reducing itsthickness.

Some of the advantages arising from the system and the method for theproduction of tubes according to the invention will be described below.

FIG. 4 shows schematically, in the form of a thickness/diameter diagram,the various types of tubes which can be produced by means of the systemaccording to the invention. In particular, three classes of tube havebeen defined in this diagram.

A first class is that included in the area denoted by A, representingtubes with a small wall thickness and a medium-to-small diameters. Asecond class is that included in the area denoted by B, representingtubes with large diameters and any wall thickness. A third class is thatincluded in the area denoted by C, representing tubes withmedium-to-large wall thicknesses and medium-to-small diameters.

The C class of tubes is the only class which may be produced withby-passing of the expanding-elongating mill and using only the singlecontinuous main rolling mill with two or more adjustable rolls forperforming rolling on a retained mandrel. As can be seen, therefore,owing to the addition of the expanding-elongating mill to the system, itis possible to broaden considerably the range of tube types which may beproduced by the system. In particular, the class of tubes denoted by Amay not be obtained by means of the main rolling mill alone because itrequires a significant reduction in the wall thickness with regard tothe pierced blank leaving the piercer. On the other hand, the class oftubes denoted by B may not be obtained by means of the main rolling millalone because it requires a significant expansion of the diameter of thepierced blank leaving the piercer.

As described above, the system and the method according to the inventionenvisage the use of continuous casting billets. These billets offer,compared to the ingots conventionally used for the production oflarge-diameter tubes, a number of significant advantages. First of all,the billet steel is of a more uniform, more controlled and, generally,superior quality. Furthermore, the cost of billets is about 30% lessthan the cost of ingots.

A main advantage, resulting from the system and method according to theinvention, is the significant reduction of the production costs. Asmentioned in the introduction, the prior art involved the expander beingfed with finished tubes stored in warehouses. In the systemconfiguration according to the invention, on the other hand, the initialblank is obtained immediately before from a billet. Since the tubes arenot required to remain for a long time in the furnace in order to heatit from room temperature to the working temperature, the problem ofoxide formation inside the tube is avoided. Moreover, working on thepiercer results in a substantial increase in the internal temperature ofthe blank, due to the friction and energy released in the form of heatduring breakage of the material. This therefore results in twosubstantial advantages: the material inside the blank remains exposed tothe atmosphere for a minimum period of time and the temperature insidethe blank, which is the most difficult to increase inside the furnace,is even greater than the outside temperature. In addition to thesubstantial reduction in energy and working time, there is also thelower cost of the billet compared to an ingot, as already mentionedabove.

A further reduction in the costs arises from the total elimination ofintermediate storage of the tubes, resulting in significant savings fromthe point of view of investment, space, operating costs and maintenance.

Finally, the elongation operations performed in the main rolling millare of a limited nature and therefore the amount of tube waste (frontend and tail end) is minimal compared to that which occurs in othermachines, such as the pilger mill. The high yield of the materialtherefore reduces production costs. Incidentally, the limited nature ofthe elongation operations also means that the stresses are very small,such that the apparatus is subject to less wear.

Compared to the prior art, the system and method according to theinvention are also able to offer substantial advantages in terms of thequality of the finished tube. The superior quality of billet steelcompared to that of an ingot has already been mentioned above. Moreover,the much smaller formation of oxides achieved with the working processaccording to the invention results in a distinctly superior workabilityof the material and therefore a better final quality. Finally, withelimination of the pilger rolling process, an improved surface qualityof the semi-finished blank—and therefore of the finished tube—isachieved, along with much smaller dimensional tolerances.

Moreover, since in the system according to the invention theexpanding-elongating mill is positioned at the start of the productionline, the subsequent machining operations manage to reduce substantiallythe problems associated with the use of this machine. In particular, themain rolling mill is able to smooth out the helical scoring whichtypically is present on the inner wall of the tube at the end of theexpander working operation. Owing to this characteristic feature of theinvention, it is possible to obtain tubes of distinctly superior qualitycompared to those obtained using conventional technology. Specialstudies carried out by the applicant have defined the internal qualityof the tubes produced according to the invention as being “very high”.Similar studies carried out on tubes of the same type, but producedusing a pilger mill or a conventional expanding mill, have defined thequality of these tubes as being “medium to low”.

It should also be mentioned that the studies conducted by the applicanthave clearly highlighted the superior concentricity of the wallthickness (i.e. the uniformity thereof along the circumference of thetube) obtainable with the system according to the invention compared toplants of the known type, in particular the pilger mill and conventionalexpander.

Considering, for example, the large and extra-large wall thicknesses,the percentage tolerances in terms of the concentricity are about halfof those obtained with a pilger mill and slightly more than half thoseobtained with a conventional expander. This qualitative advantagediminishes slightly with a reduction in the wall thickness, but remainsat percentage tolerance values significantly lower than those of theprior art.

In terms of safety for the environment and for the operators, the verylimited formation of oxides reduces to a minimum the problems associatedwith eliminating the oxides and the consequent use of a saline solution.

Finally, the system according to the invention is also characterized bya certain flexibility. In fact, this system according to the inventionallows not only the production of large-diameter tubes, i.e. with adiameter of between 18″ and 28″, but also, with by-passing of theexpanding-elongating mill, the production of medium-diameter tubes, i.e.with a diameter of between 13.⅜″ and 20″, and also of large-thicknesstubes. The production is extremely competitive in terms of quality ofthe finished tube and allows the overall productivity of the system tobe increased. Large-diameter tubes represent, in fact, only a relativelysmall share of the market and combining it with the production ofmedium-diameter tubes enable to speed up significantly amortization ofthe entire system and the return obtained from the correspondinginvestment made.

As will be clear to the person skilled in the art, the system and themethod according to the invention overcome at least partly the drawbacksmentioned with reference to the prior art.

With regard to the embodiments of the system and method for theproduction of large-diameter seamless tubes according to the invention,the person skilled in the art may, in order to satisfy specificrequirements, make modifications to and/or replace elements describedwith equivalent elements, without thereby departing from the scope ofthe accompanying claims.

What is claimed is:
 1. A system for rolling a seamless tube, the systemcomprising: a furnace for heating a billet, the billet produced bycontinuous casting; a piercing mill for longitudinally piercing thebillet to produce a pierced blank; an expanding-elongating mill forexpanding the diameter of the pierced blank; the expanding-elongatingmill for elongating the pierced blank to produce a semi-finished tube; acontinuous main rolling mill for rolling the semi-finished tube with aretained mandrel, wherein the continuous main rolling mill includes afirst plurality of rolls, each of the first plurality of rolls havingadjustable radial positions; a extracting-reducing mill positioneddownstream to the continuous main rolling mill, wherein theextracting-reducing mill includes a second plurality of rolls, each ofthe second plurality of rolls having a fixed radial position, furtherwherein the extracting-reducing mill is configured to extract thesemi-finished tube from the mandrel and reduce the diameter of thesemi-finished tube to a value that is proximate to a first desired finaldiameter of a completed version of the semi-finished tube; a sizing millpositioned downstream to the extracting-reducing mill, the sizing millincluding a third plurality of rolls, each of the third plurality ofrolls having adjustable radial positions, the sizing mill configured tocalibrate the diameter of the semi-finished tube to a value that isproximate to a second desired final diameter of the completed version ofthe semi-finished tube; and a cooling bed; wherein the system includes aby-pass line for directing the pierced blank produced in the piercingmill to the continuous main rolling mill, whereby theexpanding-elongating mill is not utilized for rolling the seamless tube.2. The system according to claim 1, wherein the piercing mill includesan ogive-shaped plug and a fourth plurality of rolls, each of the fourthplurality of rolls rotating about their respective axes in a radialposition relative to a rest of the fourth plurality of rolls.
 3. Thesystem according to claim 1, wherein the expanding-elongating millincludes an ogive-shaped plug and a plurality of variable-conicityrolls, each of the plurality of variable-conicity rolls rotating abouttheir respective axes in a radial position relative to a rest of theplurality of variable-conicity rolls.
 4. The system according to claim1, wherein a function of the piercing mill and a function of theexpanding-elongating mill can both be performed in a general mill, thegeneral mill including an ogive-shaped plug and a fifth plurality ofrolls, each of the fifth plurality of rolls having adjustable radialpositions, the ogive-shaped plug having an adjustable form, wherein thegeneral mill has a plurality of configurations based on the radialpositions of the fifth plurality of rolls and the form of theogive-shaped plug, wherein a first particular configuration of thegeneral mill can be utilized to perform the function associated with thepiercing mill, further wherein a second particular configuration of thegeneral mill can be utilized to perform the function associated with theexpanding-elongating mill.
 5. The system according to claim 1, whereinthe continuous main rolling mill includes a plurality of rolling stands,each rolling stand including a sixth plurality of rolls.
 6. The systemaccording to claim 1, wherein the continuous main rolling mill includesa plurality of rolling stands arranged in succession to each other. 7.The system according to claim 1, further comprising, positioneddownstream to the extracting-reducing mill, a means for measuring thewall thickness of a given tube; and a means for adjusting the radialposition of each of the first plurality of rolls in the continuous mainrolling mill according to a measured wall thickness of the semi-finishedtube produced by the extracting-reducing mill.
 8. The system accordingto claim 1, wherein the mandrel is held in the main rolling mill suchthat none of the mandrel's sections are subjected to a action of twosuccessive rolling stations.
 9. The system according to claim 1, whereinthe mandrel in the main rolling mill includes at least two sections andwherein a joint connecting two sections of the mandrel is not subjectedto any rolling in the first plurality of rolls.
 10. The system accordingto claim 1, the sizing mill further comprised of means for measuring atemperature of a given tube; means for measuring a diameter of the giventube; and means for adjusting the radial position of each of the secondplurality of rolls according to the temperature and diameter of thegiven tube.
 11. A method for rolling a seamless tube, the methodcomprising: heating a billet, the billet produced by a continuouscasting; longitudinally piercing the heated billet to produce a piercedblank; expanding the pierced blank; elongating the pierced blank, theexpansion and elongation of the pierced blank resulting in asemi-finished tube, the semi-finished tube having a larger diameter thanthe pierced blank, the semi-finished tube having a longer length thanthe pierced blank, the semi-finished tube having a lesser thicknessthank the pierced blank; rolling the semi-finished tube with a mandrelin a main rolling mill, wherein the main rolling mill includes a firstplurality of rolls, each of the first plurality of rolls havingadjustable radial positions, wherein the mandrel is held in the mainrolling mill such that none of a sections of the mandrel are subjectedto the action of two successive rolling stations; extracting thesemi-finished tube from the mandrel; reducing the diameter of thesemi-finished tube to a predetermined value in a sizing mill, thepredetermined value being proximate to a desired final diameter of afinished tube, wherein the sizing mill includes a second plurality ofrolls, each of the second plurality of rolls having adjustable radialpositions; and cooling the finished tube, wherein the steps of piercingthe billet and expanding and elongating the pierced billet can beperformed by a general mill, the general mill having a plurality ofconfigurations, wherein a first particular configuration of the generalmill can perform the step of piercing the billet, wherein a secondparticular configuration of the general mill can perform the steps ofexpanding and elongating the pierced billet.
 12. The method according toclaim 11, the method further comprising: measuring a wall thickness ofthe semi-finished tube extracted from the mandrel; and adjusting theradial position of the first plurality of rolls according to themeasured wall thickness.
 13. The method according to claim 1, the methodfurther comprising: measuring a temperature of the semi-finished tubeentering the sizing mill; measuring a diameter of the completed versionof the semi-finished tube leaving the sizing mill; and adjusting theradial position of the second plurality of rolls according to thetemperature of the semi-finished tube entering the sizing mill andaccording to the diameter of the completed version of the semi-finishedtube leaving the sizing mill.